feat: allow objects with no ids to be added to an empty collection

[digitalpardoe]

Whilst issues caused by the current functionality, angular/angular#22623 or #162 for example, can be solved through the use of a custom genId it strikes me that most developers who use in-memory-web-api will:

1. Be sufficiently experienced or dealing with a level of complexity from the start that they already generate IDs using genId
2. Have made the conscious decision to allow in-memory-web-api to generate their IDs
3. Be inexperienced enough (in the case of the tutorial followers especially) that it causes unexpected issues which in turn breaks their flow as they chase down a fix

In the case of 1 this PR changes nothing and in the cases of 2 and 3 it improves the functionality and developer experience.

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[digitalpardoe]

I signed it!

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CLAs look good, thanks!

[tonivj5]

any news with this?

[wardbell]

I'm sorry but I do not see that this PR addresses the concern in issue #162 nor does it change the functionality in any way. It seems to simply spread the decision logic over two functions where one sufficed before.

Please explain what I am missing.

[digitalpardoe]

I'm closing the PR as it hasn't been addressed for six months and I'm no longer in a position where I have availability to discuss it further.

If someone else would like to pick it up then you're more than welcome.